<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        img{
            vertical-align: top;
        }
        .box{
            margin: 100px;
           width: 350px;
           height: 350px;
           border: 1px solid #000000; 
           position: relative;
        }
        .box>.small>.mask{
            display: none;
            position: absolute;
            top: 0;
            left: 0;
            width: 100px;
            height: 100px;
            background-color: rgba(255,255,0,.3);
            /* 鼠标样式 移动 */
            cursor: move;
        }
        .box>.big{
            display: none;
            position: absolute;
            top: 0;
            left: 360px;
            width: 400px;
            height: 400px;
            border: 1px solid #000000;
            /* 超出部分隐藏 */
            overflow: hidden;
        }
        .box>.big>.bigImg{
            position: absolute;
            top: 0;
            left: 0;
            width: 1600px;
            height: 1600px;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="small">
            <img src="./image/001.jpg" alt="">
            <!-- 遮罩层 -->
            <span class="mask"></span>
        </div>
        <div class="big">
            <img class='bigImg' src="./image/0001.jpg" alt="">
        </div>
    </div>
</body>
<script>
    var box = document.querySelector('.box')
    var small = document.querySelector('.small')
    var mask = document.querySelector('.mask')
    var big = document.querySelector('.big')
    var bigImg = document.querySelector('.bigImg')

    // 鼠标经过小盒子，让 遮罩层，和 big 显示
    small.onmouseover = function(){
        mask.style.display='block'
        big.style.display='block'
    }
     // 鼠标离开小盒子，让 遮罩层，和 big 隐藏
    small.onmouseout = function(){
        mask.style.display='none'
        big.style.display='none'
    }

    //  遮罩层移动
    small.onmousemove = function(event){
        event = event || window.event
        var x = event.clientX - this.offsetParent.offsetLeft - mask.offsetWidth/2
        var y = event.clientY - this.offsetParent.offsetTop - mask.offsetHeight/2


        if(x<0){
            x=0
        }else if(x> box.offsetWidth - mask.offsetWidth){
            x = box.offsetWidth - mask.offsetWidth
        }

        if(y<0){
            y=0
        }else if(y>box.offsetHeight - mask.offsetHeight){
            y=box.offsetHeight - mask.offsetHeight
        }

        mask.style.left = x +'px'
        mask.style.top= y+'px'

        bigImg.style.left= - x * (bigImg.offsetWidth/small.offsetWidth) + 'px'
        bigImg.style.top= - y  * (bigImg.offsetHeight/small.offsetHeight)+'px'
    }


</script>
</html>